The present invention relates to a piezoelectric vibration element capable of effectively preventing, without decreasing the productivity, defects that a resonant frequency of the hermetically encapsulated piezoelectric vibration element fixed within an insulating case by a silicone-based conductive adhesive is degraded over time, a piezoelectric vibrator, a piezoelectric oscillator, a frequency stabilization method, and a method of manufacturing the piezoelectric vibrator.
A surface-mounted type piezoelectric device such as a crystal oscillator having a structure that the piezoelectric vibration element is hermetically encapsulated within the insulating case, is used as a reference frequency generation source, a filter or the like in an electronic apparatus like a computer or a communication apparatus like a cellular phone, a pager or the like. And miniaturization is also required on the piezoelectric device in response to the miniaturization of these various apparatuses.
In addition, a piezoelectric oscillator as a surface mounted piezoelectric device has a piezoelectric vibration element and circuit components constituting an oscillation circuit, which are received within a concave portion formed on a top surface of an insulating case formed of ceramic or the like, and an opening of the concave portion is encapsulated by a metal lid.
The piezoelectric vibration element has a structure that metal layers constituting an exciting electrode and a lead electrode are formed on a surface of a piezoelectric substrate, and the case is hermetically encapsulated while the piezoelectric vibration element is held on an inner electrode within the surface mounted type insulating case by means of a silicone based conductive adhesive (hereinafter, it will be referred to as a silicone adhesive).
FIG. 4 illustrates a phenomenon that the resonant frequency of the crystal oscillation element hermetically encapsulated within the insulating case is decreased, and it can be seen from FIG. 4 that the frequency decrease does not rapidly occur but gradually occurs over time.
That is, the tendency of the frequency decrease is hardly found in the phase that the crystal oscillator is manufactured by a piezoelectric device maker, however, it surfaces when the piezoelectric device has put on a printed board by an assembly maker and has circulated the market.
In addition, it was confirmed that the tendency of the frequency decrease becomes severe after a reflow process for loading the piezoelectric device on the printed board by the assembly maker. In addition, in response to miniaturization of the case of the crystal oscillator due to the recent requirement of miniaturization, a frequency of the occurrence of the frequency decrease actually increases.
Various inferences are drawn about the cause of these phenomena, however, an explicit cause is not found so that the fundamental solution cannot be obtained, which is the actual circumstance.
For example, a technique for solving the phenomenon of the frequency decrease of the crystal oscillator over the elapse of years is disclosed in the JP-A-7-154187.
According to this publication, it is understood that the cause of the frequency decrease of the crystal oscillator over the elapse of years lies in an oxidation phenomenon occurring on a surface layer of a metal layer constituting an electrode, so that the technique proposes attaching an insulating layer (SiO2 layer) on the surface layer of the metal layer by a deposition or sputtering method so as to cover an entire surface of the layer or oxidizing, nitridizing, or carburizing the surface layer of the metal layer in advance to form a protective layer.
In addition, this oxidization phenomenon occurs only in a nickel portion educed up to a surface layer of a gold layer when the nickel base layer was formed below the gold layer. The gold portion except the nickel portion is stabilized so that it is not oxidized.
However, according to this related art, many problems occur as follows. That is, a strict thickness control is required when a SiO2 layer is formed to cover a surface of the metal layer by deposition or the like. The SiO2 layer is apt to be stripped because of its poor adhesiveness with the gold, and when a thickness of the SiO2 layer is increased so as to prevent the SiO2 layer from being striped from the surface of the gold, a residual stress occurs on the vibrator due to the layer stress. That is, a warpage occurs due to a temperature change to cause the temperature characteristic to be degraded. In addition, when the protective layer such as an oxide layer, a nitride layer, or a carbide layer is to be formed on the surface of the gold layer, the layer is formed only in the portion where the nickel is educed as described above, however, there exists an individual drift in an area of the portion where the nickel is educed, so that a drift occurs on the added mass of the protective layer, which thus causes the frequency to be adjusted. The protective layer is not formed in the gold layer portion where the nickel is not educed, so that the frequency decrease phenomenon over the elapse of years after encapsulation not solved due to the cause to be described later in the explanation of embodiments (that is, silicone molecules are gradually and chemically absorbed into the gold layer portion).
That is, the oxidation of the irregularly educed nickel portion on the surface of the gold layer is one of the causes of the frequency decrease, however, it has been found out that the cause is not a fundamental cause. Accordingly, the solution proposed by the above-described publication is not enough.
In addition, in order to slow down the speed of the resonant frequency decrease after hermetical encapsulation, it might be considered that silicone vapor be degassed (so-called annealing) from the insulating case prior to the encapsulation, however, the silicone vapor occurs so long as the silicone adhesive is present within the case even when the degassing was carried out, which gradually attaches to the exciting electrode layer to cause the frequency change. In addition, it might be considered that a kind of the adhesive be changed, however, the silicone adhesive is very effective in terms of satisfying an impact resistance property, so that an adhesive exceeding the performance of the silicone adhesive is not present in the actual circumstance.